This invention relates to an engine system for a motor vehicle, and more particularly to an engine system which has an internal combustion engine, a carburetor, an exhaust gas recirculation (EGR) system, an exhaust gas treatment device and a secondary air supply system.
An engine system is known which has an internal combustion engine, a carburetor, an EGR system, an exhaust gas treatment device and a secondary air supply system. With the EGR system a portion of exhaust gases resulting from the combustion in the engine is introduced into the engine intake manifold through which an air fuel mixture having an air fuel (A/F) ratio adjusted by the carburetor flows toward the engine to suppress peak combustion temperatures for lowering the probability of formation of oxides of nitrogen (NOx) during the combustion. It is known that if the ratio of the quantity of exhaust gases recirculated to the quantity of intake air admitted to the engine, i.e., EGR rate, is increased, the emission of NOx will decrease. Increasing the EGR rate, however, will cause a loss of the engine power output and since the probability of misfiring increases the engine will consume more fuel.
Air pollution resulting from the emission of oxides of nitrogen is a serious public nuisance particularly in urban areas and it is during acceleration and deceleration that occur cyclically and frequently when a motor vehicle is driven in urban areas that oxides of nitrogen are discharged to the open air in such quantities as to cause a serious air-pollution problem. However, when the motor vehicle is driven in rural areas, oxides of nitrogen are not discharged in such quantities as to cause an air-pollution problem because the quantity of oxides of nitrogen discharged is reduced during crusing which occupies a main part of driving of the motor vehicle in rural areas.
It is therefore desirable for reducing a loss of engine power output and improving fuel economy to reduce the EGR rate when the motor vehicle is driven in rural areas although when the motor vehicle is driven in urban areas the EGR rate is increased to such a degree as to reduce the quantity of oxides of nitrogen to a sufficiently low level.
Hydrocarbons (HC) and carbon monoxides (CO) contained in the exhaust gases are oxidized within the exhaust gas treatment device with the help of an air supplied from the secondary air supply system. For reducing the quantity of hydrocarbons and carbon monoxides to sufficiently low levels it is necessary to keep the temperature within the exhaust treatment device higher than the reaction temperature and it has been the common practice to set the carburetor so that an air fuel mixture to be admitted to the engine will be rich enough (A/F=12 to 13, for example) to insure sufficient reaction within the exhaust gas treatment device during acceleration and deceleration which occur cyclically and frequently when the motor vehicle is driven in urban areas because it is during cyclically and frequently occurring of acceleration and deceleration when the motor vehicle is driven in urban areas that the exhaust gas temperature drops. Setting so the carburetor, however, has caused a problem that when the motor vehicle is driven in rural areas an air fuel mixture to be admitted to the engine will become excessively rich for crusing that occupies a main part of driving of the motor vehicle in rural areas, causing overconsumption of fuel during driving of the motor vehicle in rural areas thus worsening the fuel economy of the motor vehicle. Empirically it has been recognited that it is necessary to supply rich air fuel mixture when the motor vehicle is driven in urban areas in order to keep the exhaust temperature high because under this condition the exhaust temperature tends to drop to make oxidation of HC and CO difficult. However, when the motor vehicle is driven in rural areas, the exhaust temperature is high enough for sufficient oxidation of HC and CO. Thus, with lean air fuel mixture sufficient oxidation of HC and CO will result.